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of the wall, and there are no forms for Guertin to strip off after the pour. Many ICFs provide an R-value of about R-20. The Amvic 3.30 blocks chosen for the ProHOME have thicker foam than most and provide R-30 insulation and a 6-in.-wide concrete core. Even with this integrated insulation, the exterior of the footings and below-grade portions of walls on the walkout sides of the basement require an additional 21⁄2 in. of rigid EPS to circumvent heat loss. While ICFs use less concrete, they require more steel than conventional foundations, with horizontal and vertical rebar at 24 in. and 16 in. on center, respectively. Even with the rebar, the engineer on this project was concerned about the soil pressure from the north and west sides without backfill on the other sides. His concern related to potential hingepoints, weaknesses at connections of different assemblies— in this case, at the junctions of wood and ICF walls. The original plan was to stick-frame the walkout portions of the basement to “future proof” the basement by providing flexibility for window arrangements and door locations. To allay the engineer’s concerns, however, the plan was modified to use ICF construction everywhere in the foundation except for a small area around the windows on the south side. ICFs require special treatment on both the exterior and the interior because of their foam faces. Inside, code requires that a thermal barrier or ignition barrier be installed over the foam. Here, it will be CertainTeed drywall. On the exterior, Guertin will use a belt-andsuspenders approach to protect the foam. First, he will cover it from the top of the ICF down over the footings with ProtectoWrap’s Protecto Universal Primer-Free Membrane, which serves as an ICF waterproofing membrane. Next, he will install a dimple mat. This sheet provides drainage space, eliminating hydrostatic pressure, and serves to protect the waterproofing membrane from damage during backfilling. Slab details for a continuous vapor barrier In a high-performance house, Joe Lstiburek of Building Science Corporation recommends insulating basements north of the Mason-Dixon Line with R-10 subslab insulation and R-20 foundation insulation. The ProHOME’s ICFs exceed his basement guideline. To insulate the 4-in.-thick concrete slab, Guertin is using 21⁄2 in. of EPS rigid foam, which provides R-12.5. The Amvic Silverboard under the slab has a polypropylene lamination on both sides that improves durability in this application. A 6-mil poly vapor-and-gas barrier goes over the foam. All seams are taped, and the ends of the poly sheets turn up the ICFs and are taped as well. It’s important that the insulation and vapor barriers be continuous, so slab penetrations have to be considered carefully. The footings beneath the basement columns aren’t continuous with the slab. By forming the top of the footings 21⁄2 in. below the bottom edge of the slab, there is room to continue the subslab foam over the footings and to tape the vapor barrier to the columns. A good gravel base ensures that the slab performs as intended. The insulation rests on 4 in. of compacted 3⁄4-in. crushed stone. This base serves several functions: It’s easy to level, it’s a capillary break, and it provides a space for water and soil gases such as radon to move horizontally and to collect in the perforated perimeter-drain pipe. The perimeter drain carries water to daylight well past the house through a sloped drain. A vertical riser from the perimeter-drain loop passes through the slab in the northwest corner of the house and terminates through the roof. The stack effect within the riser induces a slight negative pressure in the perimeter-drain loop. In his region, Guertin has found that a passive vent and a subslab plastic vapor barrier generally control radon levels in the house. If radon levels rise to the EPA’s action level (more than 4 picocuries per liter), adding a continuously running mechanical fan in the riser should correct the problem. ProHOME SPOnSOrS The following manufacturers are supporting the construction of the ProHOME’s foundation, framing, and roof assemblies. Visit FineHomebuilding.com/prohome for a complete list of project partners and for more information on the products and materials used in ProHOME. www.finehomebuilding.com August/september 2016 71